1. Field of the Invention
This invention relates to a vacuum fluorescent printer with a print head including luminous blocks each having a plurality of luminous elements arranged in a main scanning direction for emitting, to a photosensitive material, light released by applying electrons to phosphorous objects based on a drive signal, thereby forming dots on the photosensitive material, the luminous blocks and photosensitive material being movable relative to each other in a sub-scanning direction to form images based on image data on the photosensitive material.
2. Description of the Related Art
A fluorescent printer for forming images on a photosensitive material is disclosed in Japanese Patent Laying-Open Publication H5-92622 (corresponding to U.S. Pat. No. 5,592,205), for example. This printer has cathodes for releasing thermions, grid electrodes, and a plurality of strip-like anodes covered by phosphorous objects of a predetermined size arranged at predetermined intervals, all sealed in a vacuum case. Thermion impingement upon the phosphorous objects, i.e. light emission from the phosphorous objects, is controlled by applying control signals based on image data to the grid electrodes. Each phosphorous object corresponds to one pixel of an image, i.e. one dot. The luminous blocks have numerous phosphorous objects arranged in a main scanning direction. A latent image which is a combination of numerous dots based on image data is formed on the photosensitive material by a relative movement in a sub-scanning direction (at right angles to the main scanning direction) between the luminous blocks and photosensitive material. A color fluorescent printer for printing color images includes a print head having a read (R) luminous block, a green (G) luminous block and a blue (B) luminous block. A monochromatic fluorescent print for printing monochromatic images includes a print head having a single luminous block.
In a fluorescent printer which develops and transfers to transfer paper a latent image formed on a photoreceptor drum by light dots emitted from the luminous elements synchronously with rotation of the photoreceptor drum, sensitivity characteristics of the photoreceptor drum may be maintained at a constant high sensitivity level. Where, for example, the fluorescent printer is used for exposing a photosensitive material such as photographic printing paper exposed by a light source such as a halogen lamp providing a large quantity of light, it is necessary to expose the photosensitive material over a long period of time since each phosphorous object emits light in a rather small quantity. In addition, the sensitivity characteristics are greatly variable with different types of printing paper. Printing paper with low sensitivity characteristics requires a long exposure time. This is because there is a limitation to an increase in the quantity of light based on an increase in anode voltage, and it is difficult to adjust the quantity of light only by adjusting the anode voltage. Especially in the case of color printing paper, a particular color among R, G and B could have far lower sensitivity characteristics than the other colors. When the fluorescent printer is adjusted to the low sensitivity characteristics, printing performance is greatly reduced with a prolonged exposure time.
Further, in view of the sensitivity characteristics variable with different types of photographic printing paper, it is conceivable to combine the luminous blocks with suitable filters to adjust the quantity of light. However, this would require numerous filters to produce an optimal quantity of light for each different type of printing paper with varied sensitivity characteristics, and its adjusting operation would be troublesome. A further disadvantage is that, whenever a new type of printing paper is employed, a filter suited thereto must be provided.